This invention relates to a high temperature melting composition and a method of using the composition for brazing. More specifically, but not exclusively, this invention is directed to a braze alloy, mixtures of braze alloy compositions, and methods of brazing niobium-based high temperature composites.
High temperature niobium-based alloys, such as niobium-based refractory metal-intermetallic composites (RMICs), offer combinations of superior oxidation resistance at temperatures from about 1000xc2x0 C. to about 1500xc2x0 C., good low temperature toughness, good high temperature strength and creep resistance, and desirable microstructure. Currently nickel and cobalt alloys are used for many high temperature applications. However, these alloys show signs of incipient melting at a temperature range of about 1260 to 1300xc2x0 C. It is at these high temperatures that niobium-based alloys show the greatest advantage by having good strength and creep resistance. For example, use of the high temperature niobium-based silicide composite (Nbxe2x80x94Si) alloys in advanced jet engine turbine hot sections allows the turbines to operate at higher temperatures and, consequently, allows greater power production and higher efficiency. The niobium-based alloys require new technology to fabricate turbine components including new braze technology and braze alloys to take advantage of the superior properties of the niobium-based alloys.
The high temperature niobium-based alloys typically contain one or more metals or intermetallic elements, such as aluminum, titanium, chromium, and/or silicon. There are a number of technical difficulties that must be overcome to braze niobium-based alloys containing these elements. Conventional braze alloys containing nickel, cobalt, and/or gold typically exhibit melting temperatures below about 1200xc2x0 C. Consequently, joints brazed with nickel, cobalt, and/or gold alloys have very low or no mechanical strength and oxidation resistance at temperatures above about 1250xc2x0 C.
Another drawback to the use of nickel, cobalt, and/or gold-based braze alloys is their tendency to form low melting eutectic phases in braze joints and the areas adjacent the braze joints. The eutectic phases develop from alloy interaction, and diffusion between constituents of the braze alloy and the high temperature melting substrate alloy, the niobium-based alloy. For example, nickel has a very low solubility in niobium-based alloys, since nickel and niobium atoms have very different sizes and electronic structures. When nickel is added to a niobium-based alloy, instead of forming a solid solution, nickel substitutes for some of the niobium atoms in the bulk niobium-based alloy and can combine with remaining niobium atoms to form a new nickel-niobium compound as a nickel-niobium intermetallic phase in the braze joint and/or in the area adjacent the bulk niobium-based alloy. The new nickel-niobium compound contains eutectic phases that melt at much lower temperatures than the melt temperature of the original niobium-based alloy and even the original brazed alloy. As an example, a new nickel-niobium compound containing about 76 wt % Ni and about 23 wt % Nb would have a predicted melting temperature of about 1282xc2x0 C. Similar low temperature eutectic phases would form if cobalt-based braze alloys or gold-based braze alloys were used to braze the niobium-based RMIC. The gold-based braze alloy would be particularly detrimental since it would form a low temperature eutectic phase (gold-silicon eutectic phase) that has a melting temperature of about 360xc2x0 C. The resulting braze joint and surrounding areas would then be susceptible to failure and oxidation at high temperatures. This would obviously render components made from the brazed materials useless in the high temperature sections of turbine engines.
Thus, in light of the above-described problems, there is a continuing need for advancements in the relevant field, including improved braze compositions and methods for fabrication and braze repair of high temperature melting niobium-based alloys. The present invention is such advancement and provides a wide variety of benefits and advantages.